--- title: "How we test failover: the loss window in ms | TEST4NET" description: "Failover on the CloudLAB bench: link, node and peer loss, link flap, BFD, LFA and TI-LFA, firewall HA with sessions in flight, the outage per flow in ms" image: https://test4net.com/hubfs/blog-covers/how-we-test-failover-and-convergence.png --- [Skip to content](https://test4net.com/blog/how-we-test-failover-and-convergence#main-content) [![TEST4NET](https://test4net.com/hubfs/test4net-logo-fixed-2.svg)](https://test4net.com/?hsLang=en) - Services - Products - Resources - Company - [English](https://test4net.com/blog/how-we-test-failover-and-convergence) - [Русский](https://test4net.com/ru/blog/how-we-test-failover-and-convergence) - [Հայերեն](https://test4net.com/hy/blog/how-we-test-failover-and-convergence) [News](https://test4net.com/news?hsLang=en) [Login](https://lab.test4net.com) [Support](https://test4net.com/support?hsLang=en) - [English](https://test4net.com/blog/how-we-test-failover-and-convergence) - [Русский](https://test4net.com/ru/blog/how-we-test-failover-and-convergence) - [Հայերեն](https://test4net.com/hy/blog/how-we-test-failover-and-convergence) What we test By technology Industries [Load & Performance Testing Throughput, sessions and latency up to and past the rated limit](https://test4net.com/services/load-performance-testing?hsLang=en) [Functional & Conformance Testing Features against the spec, RFC compliance with evidence](https://test4net.com/services/functional-conformance-testing?hsLang=en) [Interoperability Testing Multi-vendor behaviour, before it reaches production](https://test4net.com/services/interoperability-testing?hsLang=en) [Security & Resilience Testing NGFW, IPS and VPN efficacy and performance with threats on the wire](https://test4net.com/services/security-resilience-testing?hsLang=en) [Test Plan & Methodology Design What to test, how, and what counts as a pass](https://test4net.com/services/test-plan-design?hsLang=en) [Test Automation & CI/CD Regression suites wired into your release pipeline](https://test4net.com/services/test-automation?hsLang=en) [Training Programs Hands-on courses for your test engineers](https://test4net.com/services/training-programs?hsLang=en) [Equipment Repair Repair and verification of test equipment](https://test4net.com/services/equipment-repair?hsLang=en) [All services](https://test4net.com/services?hsLang=en) [Routing & Switching RFC 2544 and RFC 2889 on real ports, 1G to 100G](https://test4net.com/solutions/routing-switching?hsLang=en) [Firewalls & NGFW RFC 9411 throughput with inspection switched on](https://test4net.com/solutions/firewalls-ngfw?hsLang=en) [SD-WAN & WAN Path selection and QoS under emulated WAN conditions](https://test4net.com/solutions/sd-wan?hsLang=en) [Cloud & NFV Virtual appliances and cloud services under load](https://test4net.com/solutions/cloud-nfv?hsLang=en) [5G & Mobile Core Transport and core functions under subscriber traffic](https://test4net.com/solutions/5g-mobile-core?hsLang=en) [Wi-Fi Wi-Fi 6 clients and access points, Wi-Fi 7 in progress](https://test4net.com/solutions/wi-fi?hsLang=en) [Storage & Infrastructure Storage networks and servers under realistic I/O](https://test4net.com/solutions/storage?hsLang=en) [AI / Switching Fabric (RoCEv2) Lossless Ethernet for GPU clusters: PFC, ECN, fabric load](https://test4net.com/solutions/ai-fabric-rocev2?hsLang=en) [All technologies](https://test4net.com/services#coverage) [Telecom Operators Pre-deployment acceptance and vendor comparison](https://test4net.com/solutions/industries#telecom) [Network & Security Vendors Independent validation of your product claims](https://test4net.com/solutions/industries#vendors) [System Integrators Proof for the customer before the rollout](https://test4net.com/solutions/industries#integrators) [Enterprise & Banks Upgrades and new platforms checked before production](https://test4net.com/solutions/industries#enterprise) [All industries](https://test4net.com/solutions/industries?hsLang=en) Not sure where to start? 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The figure is usually taken on an idle box: one flow, two links, default timers. In production the same failure hits a router carrying 1M routes, a firewall pair holding millions of sessions, an ECMP group spreading thousands of flows, and each flow loses traffic for its own length of time. That loss window, per flow and in milliseconds, is what we measure. ## The bench The device under test sits in the CloudLAB rack on 1G, 10G or 100G ports (400G and faster per project): a [router pair or a leaf-spine pod](https://test4net.com/solutions/routing-switching?hsLang=en) or a [firewall cluster](https://test4net.com/solutions/firewalls-ngfw?hsLang=en). The generator emulates everything around it: hundreds of BGP, OSPF and IS-IS neighbours with up to 1M routes, users and servers with 2 to 20 million sessions through a firewall pair. Each flow carries sequence numbers and timestamps, so the generator sees the last packet before the failure and the first one after it. At a constant rate, lost frames divided by the rate give the outage in ms. Failures happen on purpose, at a known moment: a port shut or a fibre pulled, a neighbour stopped on the emulated side, a node powered off, a supervisor switchover, the active HA member failed. Timers, BFD, ECMP hashing, LFA and session sync come from your config export. ## What we check - Link failure: one link down with the full table loaded and traffic on every port; the gap for each flow, worst and median, then again on failback when the link returns - Node and peer loss: a router, a spine or a BGP neighbour gone; time until forwarding resumes, with BFD on and off, at your timers - Link flap: a port bounced every few seconds; convergence per event, CPU, route churn, dampening - BFD at scale: your interval and multiplier on 10 sessions and on hundreds, while the control plane is busy installing routes - LFA and TI-LFA: protected prefixes read from the box, then the real outage on covered and uncovered prefixes after the same cut - ECMP rehash: one group member lost; loss and reordering on its own flows and on flows that never touched it - Stateful failover: the active firewall fails with millions of sessions in flight; seconds until traffic flows, sessions kept, sessions that had to reconnect [RFC 2544](https://test4net.com/methodologies/rfc-2544?hsLang=en) covers a part of this with its system recovery and reset tests, and we run them when a tender asks. The rest of the list gets pass criteria from your design, written before the run. ## What usually breaks Timers. A typical acceptance: a core pair, 1M BGP routes, a design target of 500 ms. A link failure stops traffic for 1.9 s. The log shows BFD left off on one peer group and default hold timers on the other. With BFD tuned and prefix-independent convergence switched on, the rerun gives 180 ms. The datasheet number was true for the chip and said nothing about the config. Averages. LFA protects most prefixes in a ring or a partial mesh and leaves some without a loop-free backup. The median flow recovers in tens of milliseconds, the worst one waits for full convergence. ECMP has its own version: a group loses 1 link of 4, the hash is recomputed, and flows on the 3 healthy links get reordered too. A per-flow table shows both cases; a single convergence figure hides them. Sessions. An HA pair syncs its session table, and the slide promises under 1 s. In one typical run with 1 million sessions the switchover took 1.8 s, and 0.7 s after the vendor's fix. Another pattern: sessions opened in the last moments before the failure have not reached the standby yet, and they reset. On an idle pair neither shows up. ## What is in the report A table per failure type: event, affected flows or prefixes, outage for the worst and the median flow, failback, and the pass band from your design next to the measured value. For each HA failover: seconds until traffic flows, sessions kept, sessions reset. LFA coverage per prefix. The config export and the settings of the emulated neighbours, so the vendor can repeat any item. ## Where to start Send us the device models, the firmware you plan to run and the failures that worry you most. We answer with a draft test plan: the items, the bench layout and the dates. The report is yours to keep and to show to the vendor. [Write to us](https://test4net.com/contacts?hsLang=en) or look at [what else the bench does](https://test4net.com/services?hsLang=en). Share Pass it on to someone who runs a network Independent lab data is most useful in the hands of the people who make the call. Send this piece to a colleague. [LinkedIn](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Ftest4net.com%2Fblog%2Fhow-we-test-failover-and-convergence) [X](https://twitter.com/intent/tweet?url=https%3A%2F%2Ftest4net.com%2Fblog%2Fhow-we-test-failover-and-convergence&text=How+we+test+failover+and+convergence%3A+the+loss+window+per+flow) [Telegram](https://t.me/share/url?url=https%3A%2F%2Ftest4net.com%2Fblog%2Fhow-we-test-failover-and-convergence&text=How+we+test+failover+and+convergence%3A+the+loss+window+per+flow) [WhatsApp](https://wa.me/?text=How+we+test+failover+and+convergence%3A+the+loss+window+per+flow%20https%3A%2F%2Ftest4net.com%2Fblog%2Fhow-we-test-failover-and-convergence) Copy link More apps ![](https://test4net.com/hubfs/IMG_0070.jpg) Written by ### [Alexander Zemskov](https://test4net.com/blog/author/az) Alexander Zemskov leads network and security testing at TEST4NET, an independent, vendor-neutral testing lab in Yerevan, Armenia. With 19+ years in performance and load testing on IXIA, Spirent, Keysight, Xena and Xinertel platforms, he runs NGFW, WAF, IPS, DLP and SASE evaluations for telecom operators, banks and system integrators. [LinkedIn](https://www.linkedin.com/in/azemskov/) [Website](https://test4net.com/?hsLang=en) [Previous article How we test storage networks: incast, PFC and p99 at queue depth](https://test4net.com/blog/how-we-test-storage-networks?hsLang=en) [Next article How we test web portals under load: user journeys and p99](https://test4net.com/blog/how-we-test-web-portals-under-load?hsLang=en) In this article On the bench ### Have a device to put through this? Send us the shortlist and the KPIs that decide it. We will come back with a scoped test plan, a timeline and a fixed price. [Talk to a test engineer](https://test4net.com/contacts?hsLang=en) Blog ## Keep reading [![How we test hyperconverged clusters: a lost node and VM p99](https://test4net.com/hs-fs/hubfs/blog-covers/how-we-test-hyperconverged-clusters.png?width=880&height=550&name=how-we-test-hyperconverged-clusters.png)](https://test4net.com/blog/how-we-test-hyperconverged-clusters?hsLang=en) [Storage and data center](https://test4net.com/blog/tag/storage-and-data-center) ### [How we test hyperconverged clusters: a lost node and VM p99](https://test4net.com/blog/how-we-test-hyperconverged-clusters?hsLang=en) An HCI sizing sheet answers one question: how many VMs fit on 4 nodes with one node in reserve. It is calculated on a healthy cluster. A buyer reads "N+1" and expects a l… ![](https://test4net.com/hs-fs/hubfs/IMG_0070.jpg?width=22&height=22&name=IMG_0070.jpg) Alexander Zemskov Oct 30, 2026 · 5 min read [![How we test NAS over NFS and SMB: metadata calls and failover](https://test4net.com/hs-fs/hubfs/blog-covers/how-we-test-nas-file-shares.png?width=880&height=550&name=how-we-test-nas-file-shares.png)](https://test4net.com/blog/how-we-test-nas-file-shares?hsLang=en) [Storage and data center](https://test4net.com/blog/tag/storage-and-data-center) ### [How we test NAS over NFS and SMB: metadata calls and failover](https://test4net.com/blog/how-we-test-nas-file-shares?hsLang=en) A NAS datasheet gives one figure: sequential read in GB/s, taken with large files and a few clients. On a home-directory share or a build farm, hundreds of clients create… ![](https://test4net.com/hs-fs/hubfs/IMG_0070.jpg?width=22&height=22&name=IMG_0070.jpg) Alexander Zemskov Oct 30, 2026 · 4 min read [![How we test backup and replication over WAN: the window and RPO](https://test4net.com/hs-fs/hubfs/blog-covers/how-we-test-backup-and-replication-over-wan.png?width=880&height=550&name=how-we-test-backup-and-replication-over-wan.png)](https://test4net.com/blog/how-we-test-backup-and-replication-over-wan?hsLang=en) [Storage and data center](https://test4net.com/blog/tag/storage-and-data-center) ### [How we test backup and replication over WAN: the window and RPO](https://test4net.com/blog/how-we-test-backup-and-replication-over-wan?hsLang=en) The sizing sheet divides the data by the link: 20 TB over a 10G WAN is 4.4 hours, the window from 22:00 to 07:00 is 9, so the margin looks double. Over 30 ms of RTT with … ![](https://test4net.com/hs-fs/hubfs/IMG_0070.jpg?width=22&height=22&name=IMG_0070.jpg) Alexander Zemskov Oct 30, 2026 · 4 min read [![TEST4NET LLC](https://test4net.com/hubfs/test4net-logo-fixed.svg)](https://test4net.com?hsLang=en) Independent testing and validation for telecom, information security and cloud infrastructure – real insights, not assumptions [mailto:sales@test4net.com](mailto:sales@test4net.com) #### Navigate - [Home](https://test4net.com/?hsLang=en) - [Why TEST4NET](https://test4net.com/why-test4net?hsLang=en) - [About us](https://test4net.com/about?hsLang=en) - [Contact](https://test4net.com/contacts?hsLang=en) - [Blog](https://test4net.com/blog?hsLang=en) - [Industry News](https://test4net.com/news?hsLang=en) #### Get in touch Sales [sales@test4net.com](mailto:sales@test4net.com) Support [support.test4net.com](https://support.test4net.com) © 2026 TEST4NET LLC. All rights reserved. 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